The present invention relates to a developing agent used in an electronic photographing apparatus, and more specifically, to a developing agent for forming a color image.
In a full-color electronic photographing process, four colors of toners, that is, yellow, magenta, cyan and black, are developed and fixed, and thus colors including half-tones are generated. In order to reproduce an original accurately, it is required that the four colors of toners should be developed and transferred at high resolution uniformly from a half-tone region to a solid region to form an image, and half-tone colors which are obtained by mixing four colors appropriately could be expressed when fixed.
In order to be able to form a fine image, a higher resolution and a higher uniformity than those of the ordinary monochrome process are required, and in general, finer toner and finer carrier are used.
Further, in a two-component magnetic brush developing method, for a uniform development property, a developing method in which a development bias obtained by a superposition of DC and AC is often used so as to soften a brushing action of the developing agent brush is often employed. It is required that a carrier for the two-component developing agent used in this method, should be of a high resistance for the purpose of avoiding whitening in a filled section caused by leakage, or roughness on surface, or of a low maximum magnetization for the purpose of softening the brushing action.
However, when a carrier having a small particle diameter and a low maximum magnetization is used, the carrier easily adheres to the photoreceptor. Adhered carrier may be stuck between cleaning blades, and thus the photoreceptor drum is scratched, or may be stuck between the photoreceptor and the transfer roller, and thus the transfer roller is scratched. Consequently, there rise problems of disturbing a half tone of an obtained image, or creating a stripe line in a solid portion.
In the meantime, in order to obtain a color tone of a high chroma saturation, it is necessary that the transparency of the toner of each should be high and a uniform fixing surface having less roughness should be obtained. Therefore, for each toner, polyester resin having a low molecular amount, which is generally melted rapidly, is used as binder resin. However, toner which is obtained by using such binder resin has a low mechanical strength, and therefore it is difficult to maintain a sufficient life. The toner which is easy to be melted rapidly has such characteristics that offset easily occurs since the elasticity decreases while being melted. Therefore, conventionally, a mechanism which prevents offset by applying silicon oil on a fixing roller in a steady manner is generally used. However, with this method, oil easily adheres to a print matter to which an image has been fixed. In particular, when an image is fixed on an OHP sheet, there rise problems such as a stripe pattern created on the image with oil and stickiness while sheets are in storage. Further, if an oil supply mechanism is provided, the size of the overall apparatus increases, and it takes an extra work, for example, supplying oil in the tank on a regular schedule.
Under these circumstances, there has been an attempt such that the viscoelasticity does not decrease at a high-temperature state by changing the molecular weight distribution of the resin for the prevention of the offset. However, it is conventionally very difficult to achieve a high transparency of OHP as well as color generating property at the same time. Further, a so-called spent toner occurs, that is, toner sticks to the carrier or developing agent, as the toner becomes old in its life, thereby undesirably deteriorating the development property and image quality. Further, filming, that is, toner adhering to the photoreceptor, occurs undesirably.
Recently, there has been an attempt that a wax having a low melting point is mixed into the toner so as to make it unnecessary to provide an oil supply apparatus for the heat roller, thereby improving the offset property. However, this attempt entails problems of spent toner and the occurrence of filming.
As a method of preventing an increase in spent toner and improving the fluidity of toner, the addition of hydrophobic silica is widely known. However, if a great amount of hydrophobic silica is added so as to obtain a sufficient effect, a new problem that a sufficient ID cannot be obtained due to a high charge amount, arises. Further, additional problems that the difference in charge amount becoming large between a high humidity state and a low humidity state, which is caused by addition of the hydrophobic silica, and the charge amount distribution broadening at low humidity, thus increasing fogging and scattering.
Under these circumstances, a technique of maintaining an initial ID by co-using an inorganic oxide of low resistance, such as titanium oxide, as an additive, is known.
As described above, generally, a great amount of additive is used by controlling the chargeability of color toner and preventing spent toner, in order to prolong the life. However, at the same time, just because of it, the filming on the photoreceptor increases. In particular, silica having a small particle diameter easily adheres to the photoreceptor. In addition, as more toner adheres, a spot or stripe appears on the image as the toner becomes older in its life. Further, resin having a low melting point and a great amount of low-melting point wax are significant cause for the filming.
As measures for preventing filming, a technique of adding polisher such as strontium titanate, cerium oxide, aluminum oxide, silicon nitride or silicon carbide to toner so as to peel off adhered toner, has been widely used. However, with use of such a polisher, scratches on the photoreceptor increase, thus creating a rough image where white stripes and color stripes are resulted in its half tone portion.
Further, with use of the silica having a large particle diameter and a small BET specific surface, the adhesion of the toner to the photoreceptor is weakened; however at the same time, scratches on the photoreceptor increase undesirably.